I am not a lawyer but would like to share my little knowledge: (1) If the land is not TITLED, then IT IS GOVERNMENT LAND. Tax Declaration is NOT A DOCUMENT OF OWNERSHIP, but rather ONLY A DOCUMENT FOR PURPOSE OF REALTY ESTATE TAX due the government parang upa o rent for occupying the land. In short, I understand that occupying such land is only POSSESSORY Right. (2) If it is being Extra-Judicial partition, DENR should be requested. In this case, you can then have that portion applied under Miscellaneous Sale Application to DENR with the owner's Renunciation of his/her POSSESSORY rights over it conveying & transferring the rights to you. Selling or conveying/transferring right on land that is UNDIVIDED among the owner's is not advisable, it will even possible more complicated problems. (3) Strongly advise FIRST OF ALL on any transfer of untitled land is to go to DENR.
Wish you luck -- Ladie